The cost of food edged up last month - on higher tomato and fruit juice prices - but is slightly down for the year thanks to cheaper dairy products, veges and lamb.

Statistics New Zealand said food prices were up 0.1 per cent last month, compared to July, but fell 0.5 per cent for the year ending August 31.

Prices manager Chris Pike said annual food prices had now fallen for four months in a row.

"This hasn't happened since 2010, when a run of four annual falls also ended in August."

The three food items to fall the most in price during the year were lettuce - down 39 per cent, fresh milk - down 9.2 per cent, and broccoli - which fell 50 per cent.

Just two of the five food subgroups pulled the annual price change into negative territory - grocery food was down 2.1 per cent overall, while meat, poultry and fish slipped 0.7 per cent.

Lamb prices were down 19 per cent from their peak in August last year.

The food items with the biggest price increases for the year were kumara - up 90 per cent, tomatoes - up 26 per cent, and fresh chicken pieces - up 9.2 per cent.

Kumara prices hit their highest level since February 2008. The price increase was due to low prices last August and poor weather conditions affecting this year's crop.

The month on month increase in food prices was due to higher fruit and vege prices in August - thanks to a 31 per cent surge in tomato prices. Non-alcoholic beverage prices lifted 1.3 per cent, due to rises in the cost of fruit juice and soft drinks, which were not as heavily discounted compared to July.